In addition to the points already made concerning private enterprise,
it's worth bearing in mind that the tubes that were built in the
immediate wake of the City and South London Line sought to beat the
congestion in Central London caused by roads incapable of dealing with
the huge amount of horse-drawn traffic. In addition, they sought to
break into virgin suburbs in order to profit from suburban traffic.
However, the advent of the internal combustion engine lessened journey
times for all surface vehicles, making Central London sections much
less profitable, and the enormous capital sums required to build tube
lines were not compensated for by ticket receipts (railway companies
were banned from taking advantage of the appreciation in real estate
that resulted).

As a result, it became almost impossible to raise capital for tube
lines in the capital markets after about 1905. Edgar Speyer, a close
associate of Yerkes, informally broached the idea of selling the
Yerkes lines to the L.C.C. about this time, but nothing came of it.
All tube lines (and most other railway extensions, in London at least)
after 1915 and before nationalisation depended on the provision of
government assistance. This mostly consisted of loan guarantees.